Two separate developments in fundamental physics have led to the seemingly inescapable conclusion that our observable universe is part of a vast multiverse. The first development is the discovery 40 years ago of eternal inflation [1]}, [2]}, [3]}, [4]}, [5]}. It is now understood that eternal inflation is a robust phenomenon that arises for a very wide class of scalar field potentials.
The second development is the realization that string theory admits a vast landscape of metastable states [6]}, [7]}. Much remains to be understood about the string landscape, in particular the subtle constraints that quantum gravity might place on low-energy physics [8]}, [9]}, [10]}, but it seems unambiguous that it is comprised of a myriad of
metastable states, giving rise to a rich slew of effective field theories. Eternal inflation offers a mechanism to dynamically populate these vacua, resulting in the multiverse.
